Papermaking machines are divided into three dewatering sections; the forming section, the press section, and the drying section. Each section employs a papermaking fabric which serves as a dewatering and conveying medium.
The forming section receives a paper forming slurry of pulp which is approximately only 0.5% solid material. The slurry is delivered on to a forming fabric which acts to drain the water from the slurry to a point that the solid content is increased to between 18% and 23%, and slurry of pulp is formed into a sheet.
The press section receives the sheet of pulp onto a press fabric, which in cooperation with press rolls, further drains the water to increase the solid content of the sheet of pulp to between 36% and 44%.
An advanced process now employed in the press section is impulse drying. This novel process involves pressing briefly the moist sheet with a roll heated to a temperature of between 470.degree. and 700.degree. F. This process of intense heat transfer gives significantly higher dryness that wet pressing while using less energy than the conventional cylinder drying.
Normally wet felts consist of a woven base fabric formed of synthetic monofilament yarns, usually polyamide (nylon). Nylon exhibits super mechanical properties, such as good wearability, flexibility and dimensional stability. A fiber batt is normally secured to the base fabric by needling. The fiber batt is generally formed of staple length synthetic fibers such as polyesters, polyamides and in some instances polypropylene.
These synthetic fibers do not have the physical capability to operate with the high temperatures used in impulse drying. In order to overcome this disadvantage, fiber batts formed of staple fibers of a polymer of m-phenylenediamine and isophthaloyl chloride known as Nomex or an aramid known as Kelvar have been used for high temperature applications. Fiber batts formed of these synthetic materials while having the capacity to operate at high temperatures show poor mechanical properties such as: poor wearability and poor compaction resistance.
Polyaryletherketones, including polyetheretherketone, monofilaments have been employed in belts for drying ovens as illustrated by U.S. Pat. Nos. 4,359,501 and 4,820,571. Due to the excessive cost of the material, these products have not generally been accepted by industry.
It is an object of this invention to produce a papermaking press fabric which overcomes the disadvantages of the prior art as indicated above.
It is another object of the invention to provide a papermaking press felt having the elasticity characteristics and wearability characteristics of nylon so that the fabric will have superior runability on the paper machine.